In recent years, a thermally assisted recording method has been proposed as a recording method achieving a recoding density of 1 Tb/in2 or higher (H. Saga, H. Nemoto, H. Sukeda, and M. Takahashi, Jpn. J. Appl. Phys. 38, Part 1, 1839 (1999)). Conventional magnetic recording devices have a problem that information recorded at a recoding density of 1 Tb/in2 or higher may be lost due to thermal fluctuations. Although the coercivity of a magnetic recording medium needs to be increased to prevent the above problem, an excessive increase of the coercivity disables the formation of recording bits on the medium because of the limitation to the intensity of the magnetic field that the recording head can generate. To solve this problem, in the thermally assisted recording method, the coercivity of a medium is reduced by heating the medium with light at a moment of recording. This enables recording on a high-coercivity medium, and thereby achieves a recording density of 1 Tb/in2 or higher.
In this thermally assisted recording device, the diameter of a light spot for irradiation needs to be made approximately equal to a recoding bit (several tens nanometers). This is because a light spot with a larger diameter than the above erases information recorded on adjacent tracks. An optical near-field is used to heat such a small area. The optical near-field is an electromagnetic field (light having a wavenumber with an imaginary component) locally existing in the vicinity of a minute object with a diameter not larger than a light wavelength, and is generated by using a minute opening or metal scatterer with a diameter not larger than the light wavelength. JP 2001-255254 A, for example, proposes an optical near-field generator using a metal scatterer with a triangular shape as a highly-efficient optical near-field generator. When light enters the metal scatterer, a plasmon resonance is excited inside the metal scatterer, and a strong optical near-field is generated at a vertex of the triangle. With use of this optical near-field generator, the light can be highly-efficiently converged into a region of several tens nanometers or smaller. In addition, JP 2004-151046 A proposes a structure of the metal scatterer in which the surface of the scatterer on a slider air bearing surface side is partly depressed by scraping out a portion of the surface except for the vertex at which the optical near-field is generated. This structure is able to reduce the width of the intensity distribution of the optical near-field generated at the vertex and also prevent generation of a weak optical near-field (background light) at the side opposite to the vertex.